Coming down with an infection like chlamydia or gonorrhea should be a giveaway that you’ve been intimate with someone, but more than 10% of young adults who get the diagnosis won’t admit to sexual activity, according to a study published in Pediatrics. And, according to public health officials, that’s a major problem. How can we teach teens and young adults about safe sex if they won’t even admit they’re having any? (More on Time.com:5 Little-Known Truths About American Sex Lives)

Researchers tested the urine samples of 14,012 teens and young adults (average age: 22) who had filled out the National Longitudinal Study of Adolescent Health, looking for infections from Chlamydia trachomatis (chlamydia), Neisseria gonorrhoeae (gonorrhea) and Trichomonas vaginalis (trichomoniasis). When they looked at the survey answers among the group who had positive test results, they found that over 10% of that group claimed to have not had sex within the previous year — and half of those claimed to have not had sex ever.

But is it ignorance or dishonesty that is leading to the discrepancy? Is it possible these kids don’t know they are having sex, or defining sex in a different way? The New York Timesreports:

Jessica McDermott Sales, an assistant professor of public health at Emory University and one of the study’s authors, said that dishonesty was only one possible explanation for the discrepancy. But she added, “This is a fairly sizable portion of kids who are saying they’re not sexually active when they apparently are.”

The study also found that positive tests were about a third more likely in women than men, six times as likely in African-Americans as in whites, and a third more likely in those without a high school diploma than in high school graduates.

This is the first study to look at the accuracy of self-reporting among a nationally representative group of young people. Self-reporting is heavily relied upon in sexual health screening, so knowing that young people respond inaccurately to survey questions, for whatever reason, may lead researchers to re-think how they collect data in this population. And figuring out what those reasons are may be an important first step in setting up studies of adolescent sexuality in the future.